Yam RD "Blue Dream"- Ride,Maintain,Tune

[teazer]

I cannot remember the details but I think that early 250 and 350 motors had thinner heads and used 1/2" reach plugs and that 400 and late 250 motors used 3/4" reach plugs. That would explain why the long plugs are too long for a 350 head.

That is a very wide squish band and very small looking combustion chamber. Have you checked the volume?

 

[Shoeman]

I cannot remember the details but I think that early 250 and 350 motors had thinner heads and used 1/2" reach plugs and that 400 and late 250 motors used 3/4" reach plugs. That would explain why the long plugs are too long for a 350 head.

That is a very wide squish band and very small looking combustion chamber. Have you checked the volume?

Correct! E's in 400's, H's in 250/350's . And that modified chamber is very old school, squish-wise. Would be interesting to know it's volume.

 

[jpmobius]

That is a very wide squish band and very small looking combustion chamber. Have you checked the volume?

Yes, very wide squish band. Small volume chamber and good clearance (.035" - .040") will make high compression and dreadful detonation issues when the motor gets hot.

 

[2_DONE_THE_TON]

Thanks guys!

I will check with the shorter plugs and I will surely check the volume of the heads this weekend when I get back on the bike!

Am I right by doing it this way:

Put the head on a piece of glass with a piece of paper as a "gasket".Then use a syringe and fill up the head combustion chamber from above (plug thread) with a syringe until the water reaches the first thread of the plug.

 

[der_nanno]

I do it the other way round. Screw a sparkplug. Place it upside down. I normally use some plexiglass with one hole for the syringe and a second hole for bleeding out trapped air. The huge benefit of plexi glass is, that you can fix it to the with some c-clamps and so can stop everything from shifting around. Oh and plexi doesn't shatter if dropped.

:

 

[o1marc]

Never used clamps for the plexi window, normal method is to smear grease around the perimeter of the chamber and just press the plexi down tight to the surface, the grease holds it in place while you fill it with the syringe.

 

[2_DONE_THE_TON]

Hey guys!

So I had some good progress yesterday.First tried to measure the sqish band with play-doe.Then I measured the volume of the heads with some oil.
I only had a vernier calliper to measure the squish so I guess my measurement won´t be to precise...it is 2.2 MM on the squish.
The heads have a approximate volume of 23 ccm.Hope the syringe was precise enough.

Put the engine back into the frame,mocked up the exhaust,looks killer to me
I will need to make an adapter piece for the sidestand not hitting the left chamber.

 

[Shoeman]

That squish clearance, if accurate, is way too big. It's not really doing anything at that sort of gap. You're looking for 1mm or so. I can't recall what a stock 350 head should have for volume of the top of my head, it's been awhile. I'm sure somebody will chime in before I can get out to my workbench to look it up though. I know roughly what you want with ported cylinder, but that's not going to help ;D

 

[jpmobius]

yep. 1 mm. Seems like .038" is a real popular number. I am happy if I am within a couple of thousandths around the bore to that. Good to pin the heads, but best to use stock cylinder tops and machine the head to self center in the stock gasket relief and cut a groove for an o-ring. With care, you can exactly match the steps so over-torquing the now even thinner 350 heads is not an issue. You can get a good chamber volume by carefully filling the chamber with "about" the right amount of clay, assembling the engine amd carefully turning it over with the plug removed. Take it apart, carefully remove the clay and trim what extruded into the plug threads. Then submerge in a graduated cylinder and measure the displacement. You can wad it up into a ball or lozenge to fit into the cylinder. Very accurate if done carefully and takes into account the crown of the piston.

Really nice job painting/sanding those cylinders and heads.

 

[Shoeman]

Now that is a clever idea. I like ideas that let the average guy figure this critical measurement out without having to buy a burette and stand and fab plexi plates. It'll get you close enough I'd imagine if proper care is taken.

 

[der_nanno]

Now that is a clever idea. I like ideas that let the average guy figure this critical measurement out without having to buy a burette and stand and fab plexi plates. It'll get you close enough I'd imagine if proper care is taken.

I'll totally agree, this is an excellent method. Without calculating what 0.038" is in metric money, I'd say from experience slightly below 1mm is about perfect. (The number 0.6 to 0.8mm is deemed perfect in literature for the squishband to be of the biggest effect.)

After same back and forth (and the fact that my lathe is probably too small), we'll use a fly cutter on my mill to get the squishband close enough to the piston.

 

[Shoeman]

1mm = .03937" Try to stay in the .8-1mm range. .6 is too tight in my experience. Squish design and MSV calculations can be an interesting study. Also, taking a 1mm cut off that head will take very close to 3.2 cc away from it's volume. Watch the compression ratio if the motor will be on pump gas. Set the ignition timing at 1.8mm BTDC if you have not already.

 

[2_DONE_THE_TON]

Re: Yam RD "Blue Dream"- Ride,Maintain,Tune

Thanks guys for your expertise!

Ignition will be interesting,my RD has a stock CDI.will this cause any problems,since the 250 has the same stroke as the new 350 my money was on leaving the timing as it is (or was).

 

[jpmobius]

Yes, .6 mm will be too close. Ignition timing will be fine stock, but you can fool around with it after you get going. Expect to increase the chamber volume once the squish is good. Not too sure about the characteristics of the pipes (I actually own a set - just never had an occasion to use them) but often you need shockingly low (if you are a four stroke guy) to get it to not detonate itself to death when it gets hot on the street. Pipes are a big part of that picture.

 

[Shoeman]

What are those pipes, DG's? I'm not really up on the Euro ignitions, our air cooled RD's have points and the LC never made it south of the Canadian border officially, although i owned an LC motor for a bit that never got put to use. The YPVS ignition was similar in some ways and you can slot mounting holes to allow for timing changes. I'd imagine people in your region will be well versed on modding the ignition timing. It's a valuable mod to know.
If those are DG's, they are very tame but nice street pipes. You'll like them for use on an otherwise stock motor.

 

[teazer]

2.2mm sounds like a really big gap. are you sure the barrels were down tight? If that's correct, you will need to check how low the piston crowns are at TDC. If they are low, the barrels will need to be machined to bring the piston crown closer to a zero deck height.

1.2mm is a good place to start with the squish band. and get the angle right and width correct to get the appropriate MSV at the desired revs. If squish is too tight or too wide, MSV gets too high and limits revs.

 

[Shoeman]

As a more precise method IMHO, get some solder and use that instead of clay. The biggest we can get here is .062 (1.58mm) and that works well. Run the piston up close to TDC but not all the way, unroll 5 or 6 inches (sorry) put a slight bend in it, and feed it in the plug hole. You'll fell it slide along the piston done and then hit the cylinder wall. Practice if you need to. Once you're confident it's touching the cylinder wall, run the piston up past TDC. You'll feel it hit the solder. Pull the solder out and you'll see it flattened. Measure that section. You can cut that small flattened section off and do the same measurement again at four different points around the circumference of the head. The measurements will vary as probably already discussed in this thread due to mis-alignment issues with the engines design, but don't worry about that yet, you're not building a race motor here. You'll get a good feel for the average squish clearance this way and you can make changes accordingly. If the 1.58 gauge solder is not big enough to be squished by the test then using your clay measurements as a reference, take a small cut or cuts off the head and check clearance again with the solder method until you get can get a measurable squish in the solder. Fine tune the clearance from there.
With a stock chamber design, set it at 1mm and don't worry about MSV or redesigning anything. We did not for decades and stock ported street motors ran just fine set up that way. If you want to further refine it, that is a valuable aspect to refine and there is probably freeware out there now to help you do so. Way back when I bought that and several other programs from TSR, now gone, that greatly aided in designing cylinders and heads. I have not fired those up in years, probably won't even run on current windows machines. I wonder where those are? I should try and run them for fun.

 

[2_DONE_THE_TON]

Its a great pleasure to have so many wise people helping me out!

I will remeasure my combustiom volume once again with the clay method,this will be interesting since the piston crown slightly goes over the top of the cylinder gasket surface.i will let you know the result!

I will also remeasure my squish band in assistance with my friend Der Nanno who will,if needed machine the heads to the right squish clearance.

However first i will need to figure out how to reposition my sidestand,its currently hitting mte left chamber,just found a repositioning kit on HVC Cycles,maybe i can knock something out on my own

 

[der_nanno]

I don't think we can take more than 1mm off the head anyway, so going for 0.6 or 0.8mm is pretty much academic. From my past experience, a chamber volume of a tad less than 20cc should yield nice compression without terrible detonation woes. Especially considering how this bike is going to be used, there's absolutely no point in building a proper race engine for Mr. Ryan (which is also illustrated in the pistons selected etc.!)

Cheers,
Greg

 

[Shoeman]

Out of curiosity, can you measure the step in the head (gasket surface to the squish surface edge in the chamber) and let us know what that is? It's rare, but occasional heads are found that are abnormally far out of production tolerance.